Environmentally friendly fuels (e.g., alternative fuels to hydrocarbon based energy sources) are currently of great interest. One such fuel is borohydride, which can be used directly as an anodic fuel in a fuel cell or as a hydrogen storage medium (e.g., hydrogen can be liberated by the reaction of sodium borohydride with water, which produces sodium borate as a byproduct). As with all fuels, borohydride must be manufactured from readily available materials. Thus, there is a need for improved and energy efficient industrial scale manufacturing processes for producing borohydride compounds.
Typical industrial processes for the production of sodium borohydride are based on the Schlesinger process (Equation 1) or the Bayer process (Equation 2), which are both described below. Equation 1 illustrates the reaction of alkali metal hydrides with boric oxide, B2O3, or trimethoxyborate, B(OCH3)3, at high temperatures (e.g., ca. 330 to 350° C. for B2 O3 and 275° C. for B(OCH3)3). These reactions, however, provide poor molar economy by requiring four moles of sodium to produce one mole of sodium borohydride.4NaH+B(OCH3)3→3NaOCH3+NaBH4  (1)Na2B4O7+16Na+8H2+7SiO2→4NaBH4+7Na2SiO3  (2)
The primary energy cost of these processes stems from the requirement for a large excess of sodium metal (e.g., 4 moles of sodium per mole of sodium borohydride produced). Sodium metal is commercially produced by electrolysis of sodium chloride with an energy input equivalent to about 17,566 BTU (18,528 KJ) per pound of sodium borohydride produced. In contrast, the hydrogen energy stored in borohydride is about 10,752 BTU (11,341 KJ) of hydrogen per pound of sodium borohydride. The Schlesinger process and the Bayer process, therefore, do not provide a favorable energy balance, because the energy cost of using such large amounts of sodium in these reactions is high compared to the energy available from sodium borohydride as a fuel.
Furthermore, in view of the large quantities of borohydride needed for use e.g., in the transportation industry, these processes would also produce large quantities of NaOCH3 or Na2SiO3 waste products. Since these byproducts are not reclaimed or reused, further energy and/or expense would need to be expended to separate and dispose of these materials.
Typical improvements of the prior art describe simple modifications of the two processes given in equations 1 and 2. Accordingly, such improvements also suffer from the disadvantages stated above, and do not provide any improved energy efficiency. Furthermore, with the widespread adoption of borohydride as a source of hydrogen, a recycle process that allows regeneration of borohydride from borate is attractive. Thus, borohydride can be used as a fuel, and the resulting borate can then be recycled back to generate borohydride. Such a process cannot rely on the same sodium stoichiometry shown in the current borohydride manufacture processes, e.g., the Schlesinger process of Equation 1 or the Bayer process of Equation 2.